Conventional examples of transparent optical materials include methacrylic resins typified by a homopolymer of methyl methacrylate (PMMA), a polystyrene (PS), a styrene/methyl methacrylate copolymer (MS) and a polycarbonate (PC). In particular, methacrylic resins have been applied to industrial fields of signboards, lighting covers, automotive products, decorative articles and the like because they excel in transparency, surface hardness, weather resistance and the like, and have good shaping workability. Moreover, because of a small birefringence as an optical characteristic, methacrylic resins have also been applied as optical resins for optical materials such as optical disks, optical films and plastic substrates.
However, in recent years, with the development of various optical products, for example, flat panel displays such as liquid crystal displays, plasma displays and organic EL displays, small infrared sensors, micro-optical waveguides, microlenses, and pickup lenses for DVD/BlueRayDiscs handling short-wavelength light, not only excellent transparency but also high heat resistance and weather resistance, and control of a birefringence such as a low birefringence or a significant retardation have been required for optical resins for optical materials.
For example, in Patent Literature 1, as a novel transparent styrene copolymer whose heat resistance is improved, a styrene copolymer made of predetermined amounts of styrene, maleic anhydride, and methyl methacrylate is described. Moreover, in Patent Literatures 2 and 3, as a copolymer having excellent heat distortion resistance and water resistance, a copolymer obtained by polymerizing a structural mixture containing methyl methacrylate, maleic anhydride and styrene at a predetermined ratio is described.
Moreover, in Patent Literature 4, as a method of manufacturing a heat resistant acrylic resin having good optical purity, a manufacturing method in which a copolymer made of predetermined amounts of a methyl methacrylate unit and an N-alkyl-substituted maleimide unit is washed by a predetermined method is described. Furthermore, in Patent Literature 5, as a heat resistant resin, a copolymer of a structural mixture containing predetermined amounts of methyl methacrylate, N-arylmaleimide and an aromatic vinyl compound is described.
However, the copolymers and the like described in Patent Literatures 1 to 5 have a problem in weather resistance or low birefringence.
Furthermore, in recent years, as a result of refinement of the above-described various optical products, in addition to the above-described characteristics, higher uniformity of optical characteristics has been required as optical materials. In particular, it becomes necessary to control a birefringence (positive/negative/zero) of an optical material or prevent birefringence distribution in an optical material.
For example, with increase in size of flat panel displays, display screens are more often viewed not only from the front side but also from oblique directions. In such a case, due to the principle of display devices, there is a problem of display color change or contrast reduction depending on the angle at which they are viewed. Optical film materials are needed in order to improve the viewing angle characteristics, and a technique of controlling a birefringence of optical films to almost zero or to significant positive or significant negative have been required.
As a result of increase in size of flat panel displays, required optical materials are also increased in size, and since birefringence distribution is generated in the optical materials due to biased external force, there is a problem of contrast reduction. In order to reduce the birefringence distribution, an optical material having a small birefringence change caused by external force, that is, a small absolute value of a photoelastic coefficient has been required (Non Patent Literatures 1 and 2).
As a technique of controlling a birefringence of PMMA, for example, Non Patent Literatures 3 and 4 disclose a methyl methacrylate/2,2,2-trifluoroethyl methacrylate/benzyl methacrylate ternary copolymer (=52/42/6 mass %). Although the copolymer can control a birefringence and a photoelastic coefficient at the same time and can make the absolute values of birefringence and a photoelastic coefficient be zero at the same time (zero-zero birefringence), there was a problem of insufficient heat resistance.
Moreover, in Patent Literature 6, a thermoplastic resin composition containing a maleimide copolymer obtained by copolymerizing predetermined amounts of a methacrylate ester structure, a maleimide structure and an aromatic vinyl structure, and a copolymer obtained by copolymerizing predetermined amounts of a methacrylate ester structure and an aromatic vinyl structure is described. Moreover, in Patent Literature 7, as a transparent heat resistant resin composition, a transparent heat resistant resin composition containing a copolymer obtained by polymerizing predetermined amounts of methacrylate esters and a copolymer obtained by copolymerizing predetermined amounts of N-substituted maleimides (substituents are aromatic derivatives), respectively, at a predetermined ratio, is described. Moreover, in Patent Literature 8, as a transparent heat resistant methacrylic resin composition, a methacrylic resin composition containing a copolymer of a structural mixture made of predetermined amounts of methyl methacrylate, N-cyclohexyl maleimide and an aromatic vinyl compound, and a polymer containing a predetermined amount of methyl methacrylate, respectively, at a predetermined ratio, is described. Moreover, in Patent Literature 9, as a thermoplastic resin composition which excels in heat resistance, water resistance and shock resistance, a thermoplastic resin composition made of a copolymer obtained by polymerizing predetermined amounts of a specific maleimide compound, methyl methacrylate and a specific methacrylate ester, and a MMA polymer containing a predetermined amount of methyl methacrylate is described.
In contrast, in Patent Literature 10, a copolymer made of 70 to 85 wt % of methyl methacrylate monomer unit and 15 to 30 wt % of an N-substituted maleimide compound monomer unit and having an intrinsic birefringence of −0.002 to +0.002 at room temperature is disclosed.